As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which are then fed to a folding unit to form the finished, e.g. substantially parallelepiped-shaped packages.
More specifically, the pillow packs substantially comprise a parallelepiped-shaped main portion; and opposite top and bottom end portions projecting laterally on opposite sides of the main portion and defining respective triangular end flaps to be folded onto the main portion.
A longitudinal sealing strip, formed when sealing the packaging material to form the vertical tube, extends along the pillow packs; and the end portions of each pillow pack have respective transverse sealing seams perpendicular to the relative longitudinal sealing strip and defining respective end tabs projecting from the top and bottom of the pack.
The end portions of each pillow pack taper towards the main portion from the respective end tabs, and are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the end flaps onto respective walls of the main portion.
Packaging machines of the above type are known, in which the pillow packs are folded to form the parallelepiped-shaped packages by means of folding units as disclosed for example in EP-A-1726526 in the name of the same Applicant.
Folding units disclosed in EP-A-1726526 substantially comprise:                a chain conveyor feeding packs along a forming path from a supply station to an output station;        a fixed elongated guide member, which is positioned facing and at a distance from the conveyor chain and cooperates cyclically with each pack to flatten respective top end portion of the pack and so fold respective tab onto such top end portion; and        folding means cooperating cyclically with each pack to flatten respective bottom end portions and so fold respective tab onto bottom end portion.        
More precisely, folding means comprise a plurality of movable plates at least partly defining relative links of chain conveyor and hinged to such relative links.
Each plate defines an impact surface which receives relative pack by tabs of relative bottom portion and rotates between a first and a second operating position.
More specifically, in the first operating position assumed by each plate along an initial portion of forming path, relative impact surface forms with axis of the relative pack, an angle of over 90 degrees so as to fold the pack in the travelling direction of packs along forming path. Differently, in the second operating position, assumed along the remaining portion of forming path, impact surface is rotated towards pack, with which it cooperates to complete folding of relative tab onto pack.
Folding unit further comprises a fixed first cam to move impact surfaces from the relative second to the relative first operating position and a fixed second cam device located immediately upstream from the supply station and intended to move impact surface from the relative first to the relative second operating positions.
Accordingly, folding action relies substantially on the energy associated to the impact between impact surface and pack bottom end.
As a consequence, folding action relies substantially on the fact that the packs are fed to the folding action at a certain speed value. In other words, folding action can be effectively performed only when the output rate of packaging machine is higher than a certain value.
A need is felt within the industry to correctly fold the tab of pack bottom end portion even when the pack speed is particularly low, so as to obtain a folding unit suitable for packaging machine having relatively low output rate.
Furthermore, a need is felt within the industry to reduce the stresses onto packs, so as to improve the overall folding quality of packages.
A need is also felt within the industry to meet the above-identified requirement with reference to packs made by a wide range of packaging materials, especially with particularly hard packaging materials.
Finally, a need is felt within the industry to easily fold different kind of packages having relative bottom tab more or less pressed onto relative main portions.